1. Field of the Invention
This invention relates to a process for producing high-strength yarns or split yarns by slitting an ultra-high molecular weight polyolefin film into tapes and longitudinally stretching the tapes at a high stretch ratio, and also relates to a high-strength, webs-crossed laminate and a process for producing thereof using a meshy web having a unidirectionally high-strength as a longitudinal or transverse web in which adjacent yarns remains connected to each other by making slits in a web intermittently or in a shape of perforated line.
2. Related Background Art
Methods for producing high-strength yarns from an ultra-high molecular weight polyolefin material which have heretofore been known, include a method comprising dissolving the polyolefin material in a solvent, forming the resulting solution into films or threads and then superstretching the films and threads (Solvent method, Japanese Patent Publication Gazette No. Sho 37-9765), a method comprising extruding the polyolefin material and wax or the like in admixture as a variation of said solvent method (Japanese Patent Application Laid-Open Gazette No. Sho 62-182349), a method comprising continuously pressing powdered ultra-high molecular weight polyolefin for superstretching (Continuous press method, Japanese Pat. Appln. Laid-Open Gazettes Nos. Hei 2-258237 and 4-49015), and a method comprising extrusion forming an ultra-high molecular weight polyolefin to obtain an inflation film and then producing high-strength yarns therefrom (Inflation film method, Japanese Pat. Appln. Laid-Open Gazettes Nos. Sho 62-104911 and 62-122736).
The solvent method is excellent in producing yarns having good performances, but the use thereof costs the dissolution and the solvent removal and is not feasible as an industrial means from the standpoint of cost.
The continuous press method does not need a solvent and enables a higher molecular weight material to be used therein as compared with the following inflation method since the former method is not restricted to extrusion forming unlike the latter inflation method, whereby it is made advantageous in obtaining high-strength products while it raises a problem as to difficulties in producing fine yarns since it can hardly produce thin films and a continuous press apparatus for carrying it out is unsatisfactory in productivity.
The inflation method is satisfactory in productivity since it is capable of extrusion forming, but it is defective in that it cannot use a high molecular weight polyolefin in an ordinary extruder for carrying it out and it cannot effect high-ratio stretching and provide high-strength products because of entanglement of the polyolefin molecules due to melting thereof at the time of extrusion forming.
On the other hand, there have heretofore been proposed a method for producing meshy webs which comprises stretching a film such as a polyethylene or polypropylene film, and a webs-crossed laminated nonwoven fabrics made of these meshy webs. Thus, such webs-crossed laminated nonwoven fabrics are produced in quantities.
These nonwoven fabrics are good in productivity as compared with the woven ones by about two figures, are lighter in weight and more dimensionally stable than conventional nonwoven fabrics and have been used as a reinforcing material paper and films.
Further high-strength and highly dimentionally stable products, however, have been sought to be developed as various reinforcing materials for civil engineering and construction industry.